There is a problem with a printed substrate with an IC mounted thereon, in which electric current that flows on the printed substrate acts as a noise source and results in the occurrence of EMI (electro magnetic interference). This EMI can cause an electronic device with this printed substrate built therein, or other devices to malfunction. For this reason, various EMI countermeasures have been made in electronic devices in order to reduce EMI to or below an allowable value.
For example, a wiring on a printed substrate with electric current flowing therethrough and a cable connected to the substrate have electromagnetic field coupling therebetween, and this coupling causes electric current to flow also through the cable so that EMI occurs due to the cable acting as an antenna (common mode radiation). As the amount and speed of electric current that flows through a signal wiring on a printed substrate increases, this common mode radiation tends to increase, compared to before.
In order to suppress this common mode radiation, there is a need for carrying out processes regarding such as the structure of the printed substrate, the characteristics of electric current that flows through the signal wiring, and the length and/or connection position of the wiring, and adding countermeasure components. However, if design changes for EMI suppression are made and/or countermeasure components are added after the manufacturing of the printed substrate, there will be a significant increase in the cost. In order to avoid this, it is important in terms of carrying out low cost designing of a printed substrate, that the electrical characteristics are estimated at the designing stage of the printed substrate, and countermeasures for EMI suppression are accordingly made based on the result.
Examples of a method for estimating common mode radiation that occurs from the printed substrate designing stage include a method of analyzing electrical characteristics based on information of the substrate structure and components to be mounted thereon. Examples of a method for analyzing electrical characteristics include an electromagnetic field analysis method such as the FDTD (finite difference time domain) method, the moment method, and the finite element method, and a circuit analysis method such as SPICE (simulation program with integrated circuit emphasis). These methods are widely used in designing printed substrates.
Here there is a problem. Specifically, with SPICE, common mode current cannot be estimated. Therefore, when estimating common mode radiation with SPICE, a special circuit model needs to be created in order to let an electric current that simulates a common mode current flow. Moreover, in order to create this special circuit model, one with knowledge of electronic circuits and electromagnetic waves needs to perform a special process. Those who do not have in-depth knowledge are unable to perform the process, and are also unable to provide assurance of sufficient analysis precision. Therefore, common mode current estimation with SPICE is extremely difficult.
On the other hand, with the electromagnetic field analysis method, which models an entire target system, it is possible to calculate electromagnetic radiation from a cable caused by common mode current. However, when modeling an entire printed substrate including a cable to calculate the radiation electromagnetic field, a vast amount of calculation cost is required in general. Generally, calculation cost and analysis precision are a trade-off. Accordingly, if calculation cost is simply lowered, the level of analysis precision is lowered. Therefore, if calculation cost is simply reduced, sufficient assurance of analysis results cannot be obtained.
For this reason, in order to predict the amount of common mode radiation from a cable, at the designing stage of a printed substrate, there is a need for a calculation method that performs an analysis of characteristics including a common mode current flowing through a cable at a required level of precision, and an analysis design system that can obtain analysis results at a required level of precision without in-depth knowledge of electronic circuits and electromagnetic waves and that enables designing of a low-EMI printed substrate based on the results.
As a known method for designing this type of printed substrate that suppresses EMI from a cable, there is a technique disclosed in Patent Document 1. In the technique disclosed in Patent Document 1, based on the layout information of a printed substrate, an electronic device, wiring, and a ground plane are converted into a model for an electromagnetic field analysis, and a distribution of electric field intensity that occurs in the close vicinity of the ground plane as the electronic device operates is calculated. By connecting the cable to a portion with low electric field intensity, EMI from the cable is suppressed. In this way, in the technique disclosed in Patent Document 1, it is reported that an electromagnetic field analysis is performed with use of an analysis model in which a printed substrate is simplified and no cable is included, and thereby a design guideline for suppressing EMI from a cable in a short period of time is obtained.
Moreover, as a known design system, there is a technique disclosed in Patent Document 2. This technique proposes an electric field intensity calculation apparatus including a model creation means that creates an electronic circuit device model for calculating an electric field intensity by setting input data. This apparatus calculates an intensity of an electric field radiated from the electric circuit device, based on analysis input data that is obtained based on a model obtained from the model creation means. This apparatus further includes: a navigation file that stores a plurality of steps including at least a step of inputting outer dimensions of the electric circuit device, and a step of inputting an analysis frequency for generating a mesh on and analyzing the electric circuit device; and a display means that sequentially displays the steps stored in the navigation file. In this apparatus, a user sets input data in a dialogical manner in accordance with the steps displayed on the display means. In the technique disclosed in Patent Document 2, as a method for calculating electric field intensity, it is proposed to employ a technique disclosed in Patent Document 3. It is reported that, with use of this technique, it is possible to obtain optimal analysis input data that is not dependent on the proficiency of an input data creator for the same analysis conditions, and efficiently calculate electric field intensity.